Preparation of fuels for supersonic aircraft

ABSTRACT

FUELS FOR SUPERSONIC AIRCRAFT PREPARED BY OLIGOMERIZING C3 AND C4 HYDROCARBONS WHICH ARE FIRST MADE FREE OF ISOBUTYLENE AND HYDROGENATING THE OLIGOMERIZATES.

P 19 57 2 Int. Cl. C101 1/04; UWc /04 US. tCll. 208I5 5 Claims ABSTRACTOF THE DISCLOSURE Fuels for supersonic aircraft prepared byoligomerizing C and C hydrocarbons which are first made free ofisobutylene and hydrogenating the oligomerizates.

CROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of our copending application Ser. No. 88,204 filedNov. 9, 1970 and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a process for theproduction of fuels for aircraft suitable for supersonic flight, even atspeed in excess of Mach 3.

Apart from some exceptional instances, hydrocarbon fuels, which must beavailable in large quantities, are generally used for supersonicaircraft for economic reasons. The fuels are usually mixtures ofrepresentatives of all classes of hydrocarbons, the proportion andconstitution of the individual types of hydrocarbon being governed to alarge extent both by the origin and by the processing of the startingmaterial. The conditions prevailing during high-velocity supersonicflight impose considerable thermal stresses on the fuels so that,besides satisfying other requirements such as calorific value, vaporpressure and low-temperature behaviour etc., the fuels used for thispurpose must show high thermal stability. Most olefins show low thermalor thermooxidation stability, whilst aromatic hydrocarbons show apronounced tendency to form carbon black because of their unfavorablecarbonhydrogen ratio. 0n the other hand, normal parafiins have to beseparated from the mixtures in order to provide the requisitelow-temperature behaviour. Accordingly, supersonic fuels should compriseolefin-free mixtures of predominantly isoparaffins.

Energy content and combustion behaviour are also factors of considerableimportance. High calorific value per litre is generally of advantagebecause considerable distances can then be flown for a given tankvolume. It is, however, possible to increase the calorific value perlitre only to a limited extent, by employing fuels of higher specificgravity, because this involves deterioration of the low-temperaturebehaviour and/or combustion behaviour. Combustion behaviour can beregarded as satisfactory when high combustion isobtained with limitedthermal radiation and limited soot formation. The smoke point andluminometer number are used for assessment'on a laboratory scale. Theluminometer number is becoming increasingly more important. It increaseswith decreasing flame radiation, i.e. decreasing formation of carbonparticles in the flame, and should be as high as possible.

Depending upon the type of aircraft and the purpose forwhich it is to beused, the properties of the fuel have to be adapted to one another inregard to the requirements imposed on thermal stability, combustionbehaviour (eg. luminometer number and smoke point) and the energy3,814,679 Patented June 4, I974 content (calorific value per litre) onthe one hand, and satisfactory low-temperature behaviour {c.g. freezingpoint and viscosity) on the other hand. Thus, it has al ready beenproposed to prepare isoparaffinic products having properties suitablefor driving jet engines by oligomerizing C -C olefin mixtures and thenhydrogenating the oligomers. Since hardly any normal paratfins arepresent in the products obtained in this way, there is no need for anadditional treatment to adjust the necessary low-temperature behaviour.However, the products obtained are not satisfactory to such an extentthat they can be used as supersonic fuels.

SUMMARY OF THE INVENTION It has now been found that especially valuablefuels for supersonic aircraft are obtained if the olefinic C and Chydrocarbons chosen for the oligomerization and hydrogenation are firstmade substantially free of isobutylene.

DESCRIPTION OF THE INVENTION carbon mixture with a low content ofisobutylene, and

hydrogenating the resulting oligomer in known manner.

It is a important feature of this new method that the olefin-containingC and C /C,-hydrocarbon mixtures used as starting materials besubstantially free from isobutylene, before they are oligomerized andthen hydrogenated in the usual way. The supersonic fuels thus obtainedexhibit optimum properties if the isobutylene content of the C-hydrocarbons introduced into the oligomerization reactor is at aminimum. Therefore, it is of particular advantage if the isobutylenecontent does not exceed about 2% by weight based on the C -olefins andpreferably does not exceed about 1%. Although higher proportions ofisobutylene may be tolerated, they should preferably remain below 10% byweight. The oligomerizates thus obtained are not only useful assupersonic fuels as such but also as mixing components. For example,valuable supersonic fuels may be obtained by mixing according to theinvention the hydrogenated oligomerizates with properly refined andhydrogenated straight-run fractions of crude oils having a correspondingboiling range, e.g. 190-250 C. The proportions of the said fractionswhich can be admixed increase with improvement in the properties of theoligomerizates, i.e. with the lowering of the isobutylene content in thehydrocarbon mixture before oligomerization.

The removal of the isobutylene may be conducted in known manner. Forexample, the isobutylene can be extracted from a (h-hydrocarbon mixtureat ambient temperature, utilizing a 50 percent sulfuric acid as asolvent. The hydrocarbon phase is separated, neutralized and Washed withwater and then used as feed stock for oligomerization. The isobutylenemay be separated from the acid phase at high temperatures and under alow pressure. On the other hand, the isobutylene may be selectivelyoligomerized in the fluid phase at a temperature of about C. and under aslightly elevated pressure, for example in the presence of a noble metalcatalyst. The di-isobutylenes and tri-isobutylenes thus obtained can beseparated from the other olefines by distillation. In this way up to 99%of the isobutylene are removed from the C,-hydrocarbon mixture.

After the removal of the isobutylene, the olefin-containing C and C /C-hydrocarbon mixtures ordinarily are oligomerized in contact with anacidic oligomerizatiou catalyst such as phosphoric acid supported on aninert carrier such as kieselguhr. When using this catalyst theoligomerization is usually conducted at a temperature hetween about 125and 260 C. at an elevated pressure, e.g. 15 atm.

The hydrogenation of the oligomerizates thus obtained is conducted inknown manner in the presence of a suitable hydrogenation catalyst suchas platinum, ruthenium, nickel and the oxides or sulfides of molybdenum,tungsten etc. These materials may be supported by silica-alumina,kieselguhr and the like. Usually the hydrogenation is conducted at atemperature of up to about 450 C. and under a pressure of up to about200 atm.

Fuels obtained in this manner have particularly valuable properties, forexample especially high thermo-oxidation stability, and they aresuperior. to fuels prepared from oligomers obtained in a similarconventional manner from starting materials which had not been freedfrom isobutylene.

A further improvement in the fuels can be obtained by removing thearomatic hydrocarbons formed in small quantities, for example byhydrogenating them together with the olefinic hydrocarbons in knownmanner as de- The following parameters were determined in order toevaluate the various fuels:

A. Thermo-oxidation stability in accordance with ASTM D 873 by aging 100ml. of fuel for 5 hours at 148.9 C. (300 F.) under an oxygen pressure of7 atms. (100 p.s.i.), and then determining the potential residue as ameasure of stability.

B. Low-temperature behaviour by measuring (i) the crystallization pointin accordance with DIN 51,782, and (ii) the low-temperature viscosity at34.4 C. in accordance with DIN 51,562.

C. Energy behaviour by determining the lower calorific value.

D. Combustion behaviour by measuring (i) the luminometer number inaccordance with ASTM D 1740-67 T, and (ii) the smoke point in accordancewith ASTM D 1322-64.

The results with these four different fuels are set out below in thetable.

TAB LE A B C D Bolling Mg./ II, KcalJ KcaL/ II, range, C. 100 m1. I, C.cst. kg. 1. mm.

scribed above. In addition to isoparaffins, the fuel thus obtainedcontains a small proportion of naphthenes.

It is also possible initially to subject the oligomer to a mildselective hydrogenation i.e. at ambient temperature and under a pressureof about 30 to 50 atm. in the presence of a catalyst containing a noblemetal as the catalytic material in order to saturate the olefins. Thearomatic hydrocarbons are then removed from the product in known manner,for example by extractive distillation. The extraction may be conductedfor example by an aqueous solution of an organic solvent such asN-methyl-pyrrolidone at ambient or near ambient temperature in a towertype extractor followed by processing the extract phase and therafiinate phase in known manner.

If desired the product can be subjected to stronger bydrogenation togive a fuel free from aromatic hydrocarbons and naphthenes. Theseproducts, which consist almost entirely of isoparaffinic hydrocarbons,have a particularly high thermostability. In some instances it can be ofadvantage for the starting product to have a high propylene content. Theusual additives can of course be added to the fuels.

EXAMPLE Several fuels were prepared in known manner as described byoligomerization and hydrogenation:

Fuel I from a C -hydrocarbon mixture containing 37.8%

by weight of isobutylene,

Fuel II from an isobutylene-free C -hydrocarbon mixture,

Fuel III from C /C -hydrocarbon mixture containing 31.3% by weight ofpropylene and 23.5% by weight of isobutylene, and

Fuel IV from a C /C -hydrocarbon mixture containing 82.8% by weight ofpropylene and 7.4% by weight of isobutylene.

What we claim is:

1. In a process for the production of fuels for supersonic aircrafts byoligomerizing an olefin-containing C or C /C hydrocarbon mixture whichcontains isobutylene and hydrogenating the resulting oligomer, theimprovement which comprises removing the isobutylene contained in thehydrocarbon mixture to less than 10% by weight based on the C -olefinsbefore oligomerizing.

2. In a process for the production of fuels for supersonic aircraft =byoligomerizing an olefin-containing C or C /C hydrocarbon mixture whichcontains isobutylene and hydrogenating the resulting oligomer, theimprovement which comprises removing the isobutylene contained in thehydrocarbon mixture to less than 2% by weight based on the C -olefinsbefore oligomerizing.

3. In a process according to claim 1, the improvement which compriseshydrogenating the oligomer selectively to saturate the olefins andremoving the aromatic hydrocarbons from the oligomer by extractivedistillation.

4. Fuel components for supersonic aircraft produced according to theprocess of claim 1.

5. Fuel components for supersonic aircraft produced according to theprocess of claim 3.

References Cited UNITED STATES PATENTS 3/1964 Kerr et al 208-15 OTHERREFERENCES HERBERT LEVINE, Primary Examiner U.S. Cl. X.R.

